Growth hormone (GH), derived largely from the anterior pituitary, regulates postnatal growth and metabolism in vertebrates in an endocrine fashion. Recent studies also suggest roles for autocrine-derived GH and for an intact GH axis in formation and behavior of cancers in animals. GH receptor (GHR) is a cell surface glycoprotein cytokine receptor superfamily member that binds GH in its extracellular domain (ECD) and activates signaling via its intracellular domain's (ICD) interaction with the JAK2 tyrosine kinase. Our recent studies of mechanisms regulating GHR availability and activation reveal exciting insights, including: 1) JAK2 association influences GHR surface presentation, stability, and trafficking; 2) reducing prolactin receptor (PRLR) levels in human T47D breast cancer cells augments GHR abundance and GH sensitivity; 3) novel, conformationally-sensitive anti-GHR ECD antibodies may be useful GH antagonists. We hypothesize: 1) JAK2 expression levels and PRLR expression levels strongly influence GH responsiveness; 2) GH-induced GHR conformational changes that underlie GH signaling are potential targets for therapeutic intervention. Our specific aims are: 1. Determine mechanisms by which JAK2 and PRLR regulate GHR processing, cel surface stability, and downregulation. We will study how GHR predimerization impacts JAK2's modulation of GHR trafficking and the role of GHR ICD tyrosine residues on GH-independent and GH-dependent GHR trafficking. Effects of PRLR expression on GHR availability, GHR-JAK2 association, and GH actions in breast cancer cells will be examined. 2. Determine in vivo efficacy of conformation-specific inhibitory anti-GHR ECD monoclonal antibodies. We will characterize inhibitory properties of two anti-GHR antibodies and Fab fragments in signaling studies, also assessing impact of PRLR expression. We will determine in vivo efficacy of these reagents to antagonize GH signaling and cancer explant growth. These studies probe important determinants of GH sensitivity. Completion will reveal mechanisms regulating cell surface GHR availability and fate and therapeutically relevant tools to modulate GH sensitivity.